KEEP TEARING UP TRANSMISSION

I HAVE 904 DODGE AUTOMATIC TRANSMISSION WITH A MANUAL VALVE BODY THAT EVERYTIME WE RACE IT THE SPLEIN BURNS UP IN IT. WE HAVE PUT A TRANS COOLER ON IT AND WE USE SYNTHETIC TRANS FLUID . WE HAVE TO RUN THE TRANS. IN 2ND GEAR BECAUSE WE HAVE A 411 GEAR IN THE REAR END FOR THE SIZE TRACK WE RUN. WE AHVE WENT THROUGH 3 TRANSMISSION . I DON'T KNOW WHAT ELSE TO DO .